Multiplicity of microsomal cytochromes P-450 was studied with the aid of monoclonal antibodies (MAbs) to 3-methylcholanthrene induced rat liver cytochrome P-450. These MAbs specifically bind and inhibit the enzymatic activity of the purified cytochrome P-450. The contribution to enzyme activity of a particular MAb-sensitive P-450 in a mixture of different types of cytochrome P-450 can be determined from the effect of MAb on enzyme activity. We are using such enzyme-inhibition assays to develop a detailed atlas of the cytochromes P-450 for different drug and carcinogen metabolism and their distribution in different strains, species and tissues. Such information will aid us in further understanding the diversity of cytochrome P-450 and its role in tissue, strain and species susceptibility to carcinogenesis.